So a couple of thoughts here.
1. Assuming from your comments that the watch is setup using the Family setup (or whatever it's called) & the watch is not attached to a phone, so it's always on either wifi or cellular. If that's the case, then I think the 13 hours is a little low, but well within the ballpark of normal. If not, then that sounds really low.
2. Cellular uses by far the most power, so the 5 hours on cellular likely used up more battery life than the 8 hours on wifi. If it were connected to a phone via bluetooth, it would use even less power, which is likely why you haven't seen the same thing for your own watches over the years. Also, battery use on cellular depends greatly on cellular strength in your area & what you are doing with it. Tracking a walk in a good cellular area will use way less battery than streaming music in a marginal area. I have Sprint & the battery life on my watch jumped dramatically when T Mo bought them & reconfigured towers to give me a better cell signal.
3. If it were me, I'd figure out exactly when the return window closes & have her play with it for a few days to see if battery life levels out. It's likely that since it was brand new, she was using it more than she would on a normal day. If you get close to the return date & it's still not working for her, then definitely return it.
4. There are things you can do to extend battery life, ranging from simple to intrusive. I'd start with the ones that she's ok with & go from there. From my experience, anything you do that uses the cellular is the biggest battery drain. I just got back from a 69 minute run (plus a roughly 10 minute car ride). I listened to a podcast in the car & listened to music while I ran. I have a 2 year old Series 5 with 83% battery left and I'm down to 51% battery life. The things that I've found to be the most useful (and not too intrusive) are to make sure that I download as many podcast & as much music overnight while it's charging. The less I have to stream the better. In the summers, I go for lunchtime bike rides & like to listen to Tour de France podcasts. Based on the time they are released, I usually have to stream them (since they come out before I can sync them to my watch overnight) and that has a noticeable impact on my battery life. If she plans to use the watch for sleep tracking then this may not work. She can also turn on theater mode to reduce the number of times the watch face turns on (which is also one of the biggest battery drains). And the more she can stay on wifi, the better her battery life will be. When I am away form my phone, I try to use the explorer face so I can see my cellular strength (and if it has automatically connected to wifi). Also, I try to top off the charge when I can, so if I have an hour long conference call, I'll put the watch on the charger for at least 30 minutes to extend battery life. If your daughter is in school, maybe she can do that when she's driving home, or when you guys sit down to eat dinner.
5. I'm not sure this is the case any more, but the larger watches seemed to squeeze out a little more battery life than the smaller ones in the past. I originally had a 38 mm watch & switched to a 42mm & my battery life increased slightly from that. I want to say I got about an hour more, but I don't recall exactly.
6. If you do decide to keep it, I'd definitely get AppleCare for it. (I don't remember if it's apple care or apple care + or even if those are both available for the watch, but whatever one you can get). One of the benefits is a battery replacement (which is actually a refurbished watch) if your battery life dips below 80% within the terms of the apple care. During the summers, I usually have to charge my watch during the work day (I go for a run in the morning & a bike ride at lunch & without the charge, my watch won't make it through till I go to bed. I'm at 83% battery remaining at 2 years. Using a watch without a phone would almost certainly get the battery lower, so you could likely pick up a refurbished watch somewhere towards the ned of your warranty. If you get towards the end & it's still above 80%, you can convert Apple care to monthly pay & keep it in place until you cross the 80% threshold. While newer watches will certainly come with new, exciting features, I feel like the SE is one that can last for many years, so getting a refurbed one 2 years from now would easily extend the usable life for another 2 years.
Looking back at what I posted, I realize it's a lot longer than I initially thought it would be. Ultimately if it doens't work for her, then send it back. But I do find my watch to be extremely useful, so I think it's worth a little bit of tinkering to see if she can make it work.